1. Field of the Invention
The present invention relates to a work transport technique by a driving roller.
2. Description of the Related Art
A roller conveyor is known as a work transport apparatus. As a method of increasing or decreasing the work transport speed, there is proposed an arrangement in which a plurality of roller conveyor units having different transport speeds are sequentially arranged. In this arrangement, when a work passes through the roller conveyor units, it is sometimes damaged by the friction between the work and the roller owing to the transport speed difference. Japanese Patent Publication No. 47-10102 discloses an apparatus that reduces the friction between a work and a roller by cutting power transfer of some driving rollers and enabling free rotation when the work passes through the roller conveyor units. Japanese Patent Laid-Open No. 9-188414 and Japanese Patent Laid-Open No. 2000-85955 disclose apparatuses that reduce the friction between a work and a roller by variably controlling the rotational speed of a roller.
Variable control of the rotational speed of a roller requires a control apparatus for speed control, which raises the cost. Therefore, driving control of the roller is preferably ON/OFF control of constant-speed driving or stop. However, this arrangement has the above-mentioned problem of the friction between the work and the roller.
To solve this, a roller that idles upon application of a predetermined load may be used as a driving roller. When the transport speed difference is large, the roller in contact with a work idles to reduce the friction. However, when stopping a work, the deceleration distance tends to be long, and the transport apparatus becomes large. In some cases, a work does not stop at a predetermined position and overruns. To prevent the overrun, some member may abut against a work at the stop position to physically stop the work. However, the roller idles, and the work may be moved back owing to a shock at the time of abutment.